If you clone any block to itself using the "replace force" parameters of the clone command nothing happens (except that the number of cloned blocks are returned as the successcount). However if you clone a player_head or any zombie, creeper, skeleton or wither skeleton skull to itself the block remains placed but a skull item pops off, creating a duplicate.

To reproduce:

Place down a skull on the ground.

Stand on the skull and run the command `clone ~ ~-1 ~ ~ ~-1 ~ ~ ~-1 ~ replace force`

Observe the dropped skull item popping out of the block.